1. Field of the Invention
The present invention relates to an image forming apparatus and a method of adjusting a charge bias.
2. Discussion of the Background
An image forming apparatus such as a copying machine, a printer, and a facsimile machine is provided with an image carrier on which an image is formed, a charger, etc. The image forming apparatus forms an electrostatic latent image on the image carrier corresponding to image information obtained through light scanning or sent from a host computer. The electrostatic latent image is developed into a visible image and then transferred onto a recording medium (e.g., sheet).
Before image forming, the charger uniformly charges a surface of the image carrier. Known charging methods include a non-contact charging method using a corona charger, etc., and a roller charging method or contact charging method in which a charging roller contacts the image carrier.
A related-art charging roller includes an elastic layer and a high-resistivity layer provided on a core metal. A voltage is applied to the core metal to allow the charging roller to charge the surface of the image carrier. The contact charging method is preferred in recent years because less ozone is generated compared with the non-contact corona method.
However, debris (e.g., toner, paper dust, etc.), which causes unevenness of charging on the surface of the image carrier, is likely to adhere to the image carrier in the contact charging method. Therefore, a non-contact roller charging method, in which discharging is induced in a tiny gap provided between the image carrier and the charging roller, has been proposed.
Methods to apply a charging bias to a charging roller include a direct current (DC) application method and an alternating current (AC) application method. In the DC application method, a DC voltage that is controlled in a constant-voltage method (constant-voltage controlled DC voltage) is used. In the AC application method, an AC voltage that is controlled in a constant-voltage method (constant-voltage controlled AC voltage) or a constant-current method (constant-current controlled AC voltage) is overlapped on a constant-voltage controlled DC voltage.
In the AC application method, it is necessary to consider changes in surface resistance of the charging roller. For example, it becomes difficult to induce discharging when the surface resistance of the charging roller increases. By contrast, when the surface resistance of the charging roller decreases, an amount of the discharge increases and deterioration of the image carrier is accelerated. Further, image failure in a high-temperature and high-humidity environment may be generated by a discharge product. Therefore, it is necessary to adjust a peak-to-peak voltage of the AC voltage according to changes in properties of the charging roller in the AC application method.
As an example, the following method of adjusting a charging bias has been proposed: When a DC voltage is applied to a charging roller, a voltage at which discharging to an image carrier starts is referred to as a discharge start voltage or charging start voltage Vth. While image forming is not performed, at least an AC voltage value having a peak-to-peak voltage lower than twice the discharge start voltage Vth is applied to the charging roller and a supplied AC value is measured. Further, while image forming is not performed, at least two AC voltage values having different peak-to-peak voltages equal to or greater than twice the discharge start voltage Vth are applied to the charging roller and supplied AC values are respectively measured. Based on the measured AC values, a peak-to-peak voltage of an AC voltage applied to the charging roller in a subsequent image formation is adjusted.
In the non-contact roller charging method, it is necessary to consider changes in size of the gap, which affects discharging.
In one method of adjusting a charge bias, constant-voltage controlled AC voltages having different peak-to-peak voltages are applied to a charging roller and a current value supplied to the charging roller is measured. The current supplied to the charging roller when a surface potential of an image carrier becomes substantially equal to a DC voltage applied to the charging roller is referred to as a saturated current value. The peak-to-peak voltage is adjusted to such a value that the current value supplied to the charging roller becomes the saturated current value (actual value).
FIG. 1 illustrates a procedure of a related-art method of adjusting the peak-to-peak voltage of an AC voltage. At S101, an AC voltage having a certain peak-to-peak voltage is applied to a charging member as a charging bias, and an AC value supplied to the charging member is measured. At S102, it is determined whether or not the measured current value is within a target range. If the measured current value is not within the target range, an AC voltage having a different peak-to-peak voltage is applied to the charging member and an AC value supplied to the charging member is measured at S103. S102 and S103 are repeated until a supplied AC value in the target range is obtained. The above-described adjustment process is performed during a warm-up time of an image forming apparatus.